Gear roll over noise is a normal condition. Over the last twenty years or so, it has become more common. I will try to explain.
Manual transmission engineers have a choice when designing the gears. Fatter but streighter teeth are stronger and will take more load, but they are noisier. Thinner and more curved teeth are quieter but are not as strong and will not take as much load.
I first became aware of gear roll over noise in the 60's when the strongest Muncie 4 speed transmissions used in Corvettes were known as "rock crushers" because the transmission sounded like you were grinding gravel. The (I can't remember if it was the M20 or M22 that was the stronger, noisier one) noisier trans was used in the L88 option which was only intended for competition use, not street use. The L88 was a monster big block that made much more horsepower than the street version. The L88 did not even have a fan shroud and would overheat on a hot day in city traffic. It was not intended to be driven under such conditions, so noise was not considered a problem.
The "rock crusher" trans shared many components with the normal Muncie trans, but differed in the gear cuts. I don't mean that the gear ratios were different, because there was a normal Muncie with the same gear ratios. Thus they were able to make a stronger trans with minimal engineering costs AND WITHOUT A SIGNIFICANT DIFFERENCE IN WEIGHT.
As the auto industry started to become more and more concerned with fuel effeciency, every component in the car went on a diet. To keep the transmissions as light as possible and still strong enough to get the job done, more and more transmissions were designed with a bit of roll over noise. US small cars were not alone in this. The Ranger manual trans used in the 80's was made by Mazda and had gear roll over noise. Many Ranger owners needed to be educated to the fact that the noise was not harmful. The European made ZF 5 speed manual trans used in F350 trucks at about the same time were even noisier. None of these are as noisy as the old Muncie "rock crusher".
So you are not alone in being concerned about the noise. You are also not alone in having a hard time accepting the explanation of experts. If you don't believe Terry, one of the most knowledgable persons on the MTX-75 transmission, I don't know who can help.
